1. Field of the Invention
The present invention relates to a method in which a valuable rare gas is separated and recovered from a mixed gas consisting of valuable rare gases such as krypton and xenon, and in particular, to a gas separation method and apparatus in which a valuable rare gas is recovered, and separated at a high purity from a mixed gas that includes mainly a valuable rare gas and nitrogen by using a pressure swing adsorption method, and then circulated and reused.
2. Description of Related Art
In the process of manufacturing semiconductor devices such as semiconductor integrated circuits, liquid crystal panels, solar panels, magnetic disks and the like, there has been used an apparatus for generating plasma in a rare gas atmosphere in order to carry out various processes for semiconductor device by the plasma.
These plasma processes are carried out follows: 1. substrates are set in a processing chamber in a nitrogen atmosphere, 2. when plasma is generated using high frequency discharge, a rare gas flows into the chamber, 3. when the substrates are taken out wherefrom, the chamber is purged by charging nitrogen gas.
Conventionally, argon has predominantly been used as the rare gas to be employed in such treatments, krypton and xenon are coming to the fore for more sophisticated applications.
Krypton and xenon are extremely expensive, since the existence ratio of them in air is very small and the process to separate them from air is complicated, so that the semiconductor manufacturing processes using the rare gases are economically feasible, only on the premise that used gases are recovered, circulated and reused.
A method and apparatus is disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-126435, in which a valuable rare gas in a nitrogen gas is continuously recovered at a high purity and in a recovery rate of 95 to 99% or more by using a pressure swing adsorption method(below, referred to as PSA).
The method disclosed in the publication combines an equilibrium pressure swing adsorption process (below, referred to as nitrogen enriching PSA) and a rate-dependent pressure swing adsorption process (below, referred to as rare gas enriching PSA). In nitrogen enriching PSA, a valuable rare gas, which is the easily adsorbable component, is removed by using the difference in the amounts of adsorbed gases at equilibrium between nitrogen and the valuable rare gas, and the nitrogen gas, which is the hardly adsorbable component, is discharged outside the system as an off-gas. In rare gas enriching PSA, the nitrogen, which is the easily adsorbable component, is removed by using the difference in adsorption rates between the nitrogen and the valuable rare gas, and the valuable rare gas, which is the hardly adsorbable component, is extracted as product gas.
In this method, the gases desorbed in the regeneration step of the nitrogen enriching PSA and the rare gas enriching PSA are all recovered in a feed gas storage tank, mixed with a feed gas, and recirculated by using a compressor (below, the gas circulated by the compressor is referred to as the circulating feed gas). Thereby, it is possible to increase the recovery rate.
In the semiconductor manufacturing apparatus that is the object of the gas separation method of the present invention, a step in which a semiconductor substrate is fed into and removed from a processing chamber and a step in which various processes are carried out using plasma are repeated in sequence. The feed and removal of the substrates are mainly carried out in a nitrogen atmosphere, while in contrast the plasma processing is carried out in a rare gas atmosphere. Thus, the flow rate and the component concentration of the gas to be separated (below, the gas discharged from the semiconductor manufacturing apparatus is referred to as the feed gas), which is the object of the present gas separation method, are always fluctuating. The required time for the feed and removal and the time during which the plasma processing is carried out differ depending on the semiconductor manufacturing processes, and the flow rate of the rare gas required for the plasma processes and the flow rate of the nitrogen used as the purge gas is not always constant. Therefore, in the gas separation method of the present invention, measures for dealing with the fluctuations in the flow rate and composition of the feed gas are necessary. In addition, the rare gas recovered by the present separation apparatus must be continuously supplied to the semiconductor manufacturing apparatus at a required flow rate and component concentration, which depends on the semiconductor manufacturing processes.
In contrast, in the conventional method described above, it is not preferable that the component concentrations in the circulating feed gas introduced into the nitrogen enriching PSA and rare gas enriching PSA fluctuate due to the influence of the flow rate and component concentrations of the feed gas. For example, increasing the nitrogen concentration in the circulating feed gas is preferable because enriching the nitrogen in the nitrogen enriching PSA becomes easy. However, enriching a valuable rare gas in the rare gas enriching PSA, becomes difficult, and the component concentration of the valuable rare gas extracted as product decreases. In contrast, increasing the component concentration of the valuable rare gas in the circulating feed gas makes enriching the valuable rare gas in the rare gas enriching PSA, become easy. However, increasing the valuable rare gas concentration becomes difficult for the nitrogen enriching PSA and the component concentration of the valuable rare gas in the nitrogen increases, which in turn means that the recovery rate of the valuable rare gas decreases. Therefore, to supply the valuable rare gas constantly at a desired flow rate and component concentration, a control mechanism that adjusts and maintains the component concentration in the circulating feed gas is indispensable.
In consideration of the problems described above, it is an object of the present invention to provide a gas separation method and apparatus that, in separating principal gas components from a feed gas that includes a plurality of components, principal gas components can be recovered at a high efficiency by adjusting and maintaining the component concentration in the circulating feed gas within a suitable concentration range, and can supply the valuable rare gas continuously at a constant flow rate and component concentration.